For decades, optical modulators were formed of lithium niobate or another optically active compound. The size and power requirements of these devices, as well as their inability to be integrated with common electronic circuits, ultimately limited their usefulness. A significant advance in the art occurred several years ago with the advent of silicon-based optical modulators. An exemplary silicon-based modulator is disclosed in U.S. Pat. No. 6,845,198 issued to R. K. Montgomery et al. on Jan. 18, 2005 and assigned to the assignee of this application. The Montgomery et al. modulator permits relatively high speed operation (in excess of 10 Gb/s) by virtue of its “overlapped”, cantilevered configuration of a doped polysilicon layer and a doped SOI layer, with a thin dielectric disposed in the overlap region. Referred to as a “SISCAP” (silicon-insulator-silicon capacitance) modulator, the overlapped configuration results in the carrier integration window essentially overlapping the optical mode, allowing for efficient modulation based upon carrier movement across the dielectric layer.
FIG. 1 is a simplified concept illustration of the Montgomery et al. SISCAP modulator (also referred to hereinafter as an SOI-based modulator). In this case, SOI-based optical modulator 1 comprises a doped silicon layer 2 (typically, polysilicon and referred to at times hereafter as the “gate” layer) disposed in an overlapped arrangement with an oppositely-doped portion of a sub-micron thick silicon surface layer 3 (often referred to in the art as an SOI layer, or the “body” layer of the modulator structure). SOI layer 3 is shown as the surface layer of a conventional SOI structure, which further includes a silicon substrate 4 and buried oxide layer 5. Importantly, a relatively thin dielectric layer 6 (such as, for example, silicon dioxide, silicon nitride or the like) is disposed within the overlapped region between SOI layer 3 and doped silicon layer 2. The overlapped area defined by silicon layer 2, dielectric 6 and SOI layer 3 defines the ‘active region’ of optical modulator 1. Free carriers will accumulate and deplete on either side of dielectric 6 as a function of the voltages applied to SOI layer 3 (V3) and/or doped silicon layer 2 (V2). The modulation of the free carrier concentration results in changing the effective refractive index in the active region, thus introducing phase modulation of an optical signal propagating along a waveguide defined by the active region (the optical signal propagating along the y-axis, in the direction perpendicular to the paper).
In a preferred embodiment of this modulator arrangement, the contacts to layers 2 and 3 are spaced from the active region of the modulator, as shown in FIG. 1. A first contact region 7 (such as a silicide region) is disposed over an outer portion of layer 2, and a second contact region 8 is similarly disposed over an outer portion of layer 3.
It has been found that an improvement in performance is achieved if the region of the layers immediately adjacent to the contacts is more heavily doped than the central regions supporting the optical mode. The heavily-doped contact areas provide a very low resistance coupling to the contact regions. FIG. 2 illustrates this particular embodiment, showing a high dopant concentration area N+ within doped gate layer 2 in association with contact region 7 and a high dopant concentration area P+ within doped body layer 3 in association with contact region 8.
The speed of this prior art SISCAP modulator is determined by the equivalent resistance (R) and capacitance (C) between contact regions 7 and 8. FIG. 3 is a typical C-V curve for the device shown in FIG. 2. Obviously, for high speed applications with a fixed capacitance C (defined by the properties of dielectric 6), it is desirable to have as low a resistance value as possible, effectuated by placing the device contacts close to the active region. While providing an increase in operating speed, however, a relatively low resistance as achieved in this fashion will increase the optical loss by having the high optical absorption contact regions very close to the optical mode of the active region. FIG. 4, which is a depiction of a relevant portion of the prior art SISCAP modulator, illustrates this problem, where a significant portion of the optical intensity is shown to reside beyond the boundary of the active region. Indeed, the optical intensity approaches the location of the heavily-doped portions of gate layer 2 and body layer 3.
Thus, a need remains for increasing the speed of the silicon-based optical modulator as shown in FIGS. 1 and 2 without introducing an unacceptable increase in optical loss, as shown in the device of FIG. 4.